1. Field of the Invention
The present invention relates to a method of manufacturing a semiconductor device, including a step of annealing a semiconductor film by using laser or equivalent strong light. The semiconductor apparatus refers to all of devices, which can function by using semiconductor characteristics and may be an electro-optical device, such as a liquid crystal display device, an electronic apparatus including an electro-optical device as its component, and a light emitting device.
2. Description of the Related Art
A technology is widely known whereby laser annealing is performed on a semiconductor film formed on an insulating substrate, such as glass, for the crystallization, the improvement of crystalline characteristics, and/or the activation of dopant added to the semiconductor film. A silicon film may be used as the semiconductor film in many cases.
A glass substrate may be used as the insulating substrate in many cases. The glass substrate can be processed into a big area substrate of, for example, 600×720×0.7 mm in size. In addition to the glass substrate, quartz substrate may be used. However, it is significantly difficult for the quartz substrate to be processed into a big area substrate. The use of the glass substrate has many advantages, but there is a problem that the melting point of the glass substrate is lower than that of the quartz substrate. Since annealing the semiconductor film requires a higher temperature, there is a problem of deformation of the glass substrate when annealed. Laser annealing of the semiconductor film was devised in order to overcome the problems. Laser can emit significantly strong energy in a short period of time. Therefore, it is possible to heat an object in an unbalanced manner. Thus, it is possible only to increase the temperature of the semiconductor film without much increase in temperature of the glass substrate. Accordingly, laser is preferably used for annealing the semiconductor film formed on the glass substrate.
The crystalline semiconductor film obtained by using the technology includes many crystal grains and therefore is called polycrystalline semiconductor film. The polycrystalline semiconductor film has much higher mobility than that of an amorphous semiconductor film. Therefore, by using the polycrystalline semiconductor film, an active-matrix type of liquid crystal display device (a semiconductor device in which thin film transistors (TFT) for driving pixels and for a driving circuit are produced on one substrate) can be produced, which cannot be achieved by using a semiconductor device produced by using, for example, a conventional amorphous semiconductor film. Accordingly, the polycrystalline semiconductor film has a much higher characteristic than that of the amorphous semiconductor film.
On the other hand, a method is devised which can be performed by using thermal processing in a lower temperature in the crystallization process of the amorphous semiconductor film. The method is disclosed in Japanese Patent Application Laid Open No. 7-183540 in detail. Here, an example of the method of crystallization of an amorphous semiconductor film by heating will be described briefly. First of all, a small amount of an element such as nickel, palladium and lead is added to an amorphous semiconductor film. A method for the addition may be plasma processing, vapor deposition, ion-implantation, sputtering and solution coating. After the addition, the amorphous semiconductor film is heated for four hours in an atmosphere of nitrogen at 550° C., for example. Then, a polycrystalline semiconductor film is obtained. A heating temperature and a heating time, which are optimum for the crystallization, depend on the amount of the added element and/or the condition of the amorphous semiconductor film.
As described above, the crystallization by laser annealing can give high energy only to an amorphous semiconductor film without much increase in temperature of the substrate. Therefore, the crystallization by laser annealing can be used not only for a glass substrate with a lower distortion point but also for a plastic substrate. Laser annealing is used for a process of activating dopant added to the semiconductor film. In many cases, thermal annealing may be performed for the process.
In the present mass-production process, laser used for laser annealing is excimer laser. The excimer laser adopts the pulse system and therefore has a large output. Further, the excimer laser has a significantly high coefficient of absorption for a silicon film, which is often used for the semiconductor film. Therefore, the excimer laser is used for the mass production process. A laser-annealing method is preferably used, including the steps of forming laser light in the pulse system with a larger output on the irradiated surface such that it can be a spot of several centimeter square or a 10 cm or longer line in an optical system and scanning the laser light (or moving a position of irradiating the laser light with respect to the irradiated surface). This is because the method results in higher productivity and is excellent for industrial purposes.
Especially, when laser light whose form on the irradiated surface is linear (called linear beam, hereinafter) is used, unlike a case where spot-form laser light requiring up/down and right/left scanning, the laser light can be irradiated onto a large area only by scanning in the direction perpendicular to the direction of the linear beam, which results in higher productivity. Scanning in the direction perpendicular to the line direction is performed because the direction is the most efficient scanning direction. Due to the higher productivity, using linear beam resulting from forming laser with a large output in the pulse system in an appropriate optical system becomes a mainstream gradually. The linear beam is especially effective in mass production processing, which uses a large area substrate of 600×720×0.7 mm in size.
It is an object of the present invention to reduce unflatness formed on a surface of a semiconductor film when annealing the semiconductor film by using laser and, at the same time, to reduce the cost for a technology of reducing the unflatness.
A semiconductor film can be crystallized and/or the crystalline characteristics can be improved by irradiating laser onto a semiconductor film. However, during the processing, the semiconductor film is melted by laser energy and then returns to a solid state again. When it returns to the solid state, the semiconductor film forms innumerable nuclei. Each of the nuclei grows mainly in parallel with a surface of the semiconductor film and then produces a crystal grain. The adjacent crystal grains collide with each other during the growth of the crystal grains. Thus, very high projecting portions are formed on the semiconductor film. Especially when the semiconductor film is annealed by using laser in an atmosphere including oxygen, such as an atmosphere of air, the projecting portion grows significantly and can reach to substantially the same height as that of the thickness of the semiconductor film. In this way, projections and depressions are formed on the surface of the semiconductor film to which laser is irradiated. However, especially when a top-gate type TFT is produced, surfaces of the projections and depressions become an interface against a gate insulating film. Therefore, the projections and depressions cause variation in element characteristics and/or increase in off current values.
It is known that the growth of the projecting portion can be suppressed significantly if oxygen is removed from the atmosphere for annealing the semiconductor film by using laser. Therefore, the annealing is often performed in an atmosphere without oxygen, such as in an atmosphere of nitrogen or in a vacuum. Thus, the unflatness of the semiconductor film may be suppressed. However, in order to have the atmosphere of nitrogen or the vacuum for the laser annealing, a robust vacuum chamber, a vacuum exhaust device and/or a large amount of nitrogen supply are required, which increases the size of the apparatus and the cost. The present invention can reduce the cause to increase the cost. In other words, it is an object of the present invention to provide a device of flattening a surface of a semiconductor film by annealing the semiconductor film by using laser in an atmosphere without oxygen, without the use of a vacuum device, and to provide a method of manufacturing a semiconductor apparatus.